1. Field of the Invention
The present invention generally relates to optical disk recording apparatuses and methods of recording data on optical disks, and more particularly to an optical disk recording apparatus that records data on an erasable optical disk and a method of recording data on an erasable optical disk.
2. Description of the Related Art
Direct-read-after-write optical disks are of two general types: write-once and erasable. In overwriting an erasable optical disk, a laser beam having a laser power alternating between a write power Pw and an erase power Pe is employed. In this case, the write power Pw forms pits on the recording film of the optical disk by changing the state of the recording film from a crystalline state to an amorphous state, and the erase power Pe erases the pits by changing the state of the recording film from the amorphous state to the crystalline state.
The Optimum levels of the write and erase powers Pw and Pe (optimum write and erase powers Pwo and Peo) of the laser beam at a time of recording data on an optical disk differ depending on a type of the optical disk, a recording apparatus, and a recording rate. Therefore, in order to set the optimum write and erase powers Pwo and Peo based on the combination of a type of the optical disk, a recording apparatus, and a recording rate employed in a practical recording, a recording power calibration called OPC (Optimum Power Control) is performed prior to the recording of data.
A description will be given of an OPC operation in a conventional optical disk recording apparatus for an erasable optical disk.
FIG. 1 is a diagram for illustrating the test recording area of the optical disk.
As shown in FIG. 1, a data area for storing a variety of data and a PCA (Power Calibration Area) that is a test recording area for setting the optimum write power of a laser beam are formed on the recording surface of the optical disk. The PCA, which is formed along a track closest to a disk center, includes a test area and a count area. The test area is formed of 100 partitions, and each partition is formed of 15 frames. In one OPC operation, one partition is used so that each frame of the partition is recorded with a test signal with a different level of the laser power. That is, the laser power has the 15 levels. Each test signal is an EFM-modulated signal formed of a pulse train having a pulse width three to ten times a reference pulse width T, and is recorded on each frame with pits of nine types of lengths. The reference pulse width T is for one cycle of a pulse signal having a frequency of 4.32 MHz, and is 230 nsec at a normal rate (1xc3x97).
The laser beam is emitted onto each frame so that each test signal is reproduced by detecting a light reflected back therefrom. At the same time, a modulation degree m as an index indicating the magnitude of the amplitude of each reproduced HF (high frequency) signal is measured by using the following equation
m=I11/Itopxe2x80x83xe2x80x83(1) 
where I11 is an amplitude of a reproduced HF signal based on pits and lands (regions between pits) of a pulse width 11 times the reference pulse width T, and Itop is the optical reflectivity of the lands as shown in FIG. 2. In FIG. 2, I3 is an amplitude of the reproduced HF signal based on pits and lands of a pulse width three times the reference pulse width T. The modulation degree m differs depending on the level of the write power Pw. As shown in FIG. 3, as the level of the write power Pw becomes lowers, the modulation degree m becomes smaller because the amplitude of the reproduced HF signal becomes smaller. On the other hand, as the level of the write power Pw becomes higher, the modulation degree m becomes larger because the amplitude of the reproduced HF signal becomes larger.
In the case of determining the optimum write power Pwo by the modulation degree m, a method using a parameter xcex3 obtained from the characteristic of the modulation degree m is employed. The parameter xcex3 is given by the following equation
xcex3=(dm/dPw)xc3x97(Pw/m)xe2x80x83xe2x80x83(2) 
That is, the parameter xcex3 is the differential of the characteristic of the modulation degree m. The optical disk is prerecorded with the target value xcex3target of the parameter xcex3 as ATIP (Absolute Time In Pregroove) information. Therefore, as show in FIG. 4, the characteristic of the parameter xcex3 is obtained from the characteristic of the modulation degree m according to the above-described equation (2), and then a write power level Ptarget that realizes the target value xcex3target is obtained. Since the optical disk is prerecorded with a coefficient xcfx81 for obtaining the optimum write power Pwo from the write power level Ptarget as the ATIP information, the optimum write power Pwo is obtained, by using the coefficient xcfx81, from the following equation
Pwo=xcfx81xc3x97Ptargetxe2x80x83xe2x80x83(3) 
The optimum write power Pwo obtained from the above-described equation (3) is set and used as a write power at the time of a signal recording. The optimum erase power Peo is set, by using the optimum write power Pwo and a coefficient xcex5 (a Pe/Pw ratio) recorded on the optical disk as the ATIP information, based on the following equation
Peo=xcex5xc3x97Pwoxe2x80x83xe2x80x83(4) 
FIG. 5 is a flowchart of a conventional OPC operation. In FIG. 5, in step S10, a recording rate is set to a value specified by a write command, and in step S12, a medium (optical disk) type is determined by obtaining an ID number recorded on the optical disk as the ATIP information.
Next, in step S14, a table storing a start power and a step power which table is prerecorded in memory is selected based on the recording rate. In step S16, the write power Pw is successively changed from the start power by the amount of the step power to have 15 different levels so that test signals are recorded in the test recording area of the optical disk.
Next, in step S18, the test signals are reproduced, and in step S20, the modulation degree m is measured with respect to each of the 15 levels of the write power Pw so that the characteristic of the modulation degree m indicated by a solid line in FIG. 4 is obtained. The characteristic of the modulation degree m is a relation between the modulation degree m and the write power Pw of the 15 levels. Then, in step S22, the parameter xcex3 is obtained with respect to each of the 15 levels of the write power Pw from the characteristic of the modulation degree m by using the equation (2). The parameter xcex3 with respect to the modulation degree m is indicated by a dot-dash line in FIG. 4.
Next, in step S24, the write power level Ptarget realizing the target value xcex3target included in the ATIP information of the optical disk is obtained from the characteristic of the parameter xcex3 indicated by the dot-dash line in FIG. 4. Then, in step S26, the optimum write power Pwo is obtained by multiplying the write power level Ptarget by the coefficient xcfx81 (a real number larger than or equal to one) included in the ATIP information, and in step S28, the optimum write power Pwo is stored in memory. Thereafter, in step S30, the OPC operation ends, and in step s32, a recording is started with the above-described optimum write power Pwo.
With respect to an erasable optical disk, since a saturated laser power deteriorates the durability of the optical disk, a laser power of a point at which a modulation degree starts to be saturated becomes an optimum laser power. However, a change in a modulation degree change rate (the parameter xcex3) is small at the point at which the modulation degree starts to be saturated. Therefore, a point at which a change in the modulation degree change rate is great (a point corresponding to a laser power smaller than the optimum power) is set as the target value xcex3target, and the optimum write power Pwo is obtained by multiplying a power of the point by the coefficient xcfx81.
The characteristic of the modulation degree changes based on an error between drives, such as a modulation degree measurement offset due to a difference between the measurement circuits of the drives, or based on a difference between disks in each of which a relation between the optimum laser power and the modulation degree differs. Therefore, if the optimum laser power is obtained from a relation between the modulation degree change rate and the laser power, the obtained optimum laser power does not satisfy the recording characteristic of each drive and the durability of each disk.
However, according to the conventional OPC operation, if an error is caused in a measurement of the modulation degree m at the time of a test recording by an unevenly formed recording film of the optical disk or a fluctuation in the movement of the signal recording surface of the optical disk in rotation, an error is also caused in the parameter xcex3 obtained from the characteristic of the modulation degree m. Thus, the optimum write power Pwo is prevented from being determined with accuracy since the optimum write power Pwo is obtained by using the parameter xcex3 including the error.
Further, both of the characteristics of the modulation degree m and the parameter xcex3 are different in the case of performing initial one hundred test recordings by using the PCA of the optical disk and in the case of performing test recordings after erasing the data of the initial one hundred test recordings from the PCA. Therefore, the optimum write power Pwo is prevented from being determined with accuracy and stability.
It is a general object of the present invention to provide an optical disk recording apparatus and a method of recording data on an optical disk in which the above-described disadvantages are eliminated.
A more specific object of the present invention is to provide an optical disk recording apparatus and a method of recording data on an optical disk which apparatus and method reduce the effect of a change in the characteristic of a modulation degree so as to determine an optimum write power level with accuracy.
The above-objects of the present invention are achieved by a recording apparatus for an erasable optical disk, which apparatus includes: a first determination part which determines a maximum of modulation degrees of signals reproduced from a test recording area of the optical disk, the signals being recorded in the test recording area with corresponding write power levels different from one another; and a second determination part which determines an optimum write power level corresponding to a target modulation degree obtained from the maximum of the modulation degrees based on a relation between the modulation degrees and the corresponding write power levels, wherein the recording apparatus records data on the optical disk with the optimum write power level.
The above objects of the present invention are also achieved by a method of recording data on an erasable optical disk, which method includes the steps of (a) determining a maximum of modulation degrees of signals reproduced from a test recording area of the optical disk, the signals being recorded in the test recording area with corresponding write power levels different from one another, (b) obtaining a target modulation degree from the maximum of the modulation degrees, (c) determining an optimum write power level corresponding to the target modulation degree based on a relation between the modulation degrees and the corresponding write power levels, and (d) recording the data on the optical disk with the optimum write power level.
According to a conventional method employing a parameter xcex3 obtained from the characteristic of the modulation degree in determining the optimum write power, if an error is caused in a measurement of the modulation degree, it is natural that an error should also be caused in the parameter xcex3, thus preventing the optimum write power from being determined with accuracy. However, according to the above-described apparatus and method of the present invention, the optimum write power is determined based on the maximum of the modulation degrees. Since the maximum of the modulation degrees is obtained around a saturation point of the modulation degree where a change in the modulation degree is about to be minimized with respect to the write power level, the maximum of the modulation degrees can be measured with stability even when the write power level is changed in an OPC operation. Therefore, by normalization using the maximum of the modulation degrees, a variation in the characteristic of the modulation degree due to a drive or disk difference can be absorbed, and an effect of a measurement error of the modulation degree in an OPC operation can be reduced. Therefore, the optimum write power can be determined with good accuracy.